With the evolving of semiconductor technologies, semiconductor dies are becoming increasingly smaller. More functions, however, need to be integrated into the semiconductor dies. Accordingly, the semiconductor dies need to have increasingly greater numbers of I/O pads packaged into smaller areas, and the density of the I/O pads rises quickly. As a result, the packaging of the semiconductor dies becomes more difficult, and adversely affecting the yield.
Package technologies can be divided into two categories. One category is typically referred to as wafer level package (WLP), wherein the dies on a wafer are packaged before they are sawed. The WLP technology has some advantageous features, such as a greater throughput and a lower cost. Further, less underfill or molding compound is needed. However, the WLP technology suffers from drawbacks. The conventional WLP can only be fan-in type packages, in which the I/O pads of each die are limited to a region directly over the surface of the respective die. With the limited areas of the dies, the number of the I/O pads is limited due to the limitation of the pitch of the I/O pads. If the pitch of the pads is to be decreased, solder bridges may occur. Additionally, under the fixed ball-size requirement, solder balls must have a certain size, which in turn limits the number of solder balls that can be packed on the surface of a die.
In the other category of packaging, dies are sawed from wafers before they are packaged onto other wafers, and only “known-good-dies” are packaged. An advantageous feature of this packaging technology is the possibility of forming fan-out chip packages, which means the I/O pads on a die can be redistributed to a greater area than the die itself, and hence the number of I/O pads packed on the surfaces of the dies can be increased.
The formation of fan-out WLP faces challenges. For example, the formation of the fan-out WLP involves various materials whose characteristics have significant difference. Accordingly, the adhesion strength at the interfaces of these materials needs to be improved. The moisture isolation of the fan-out WLP needs to be improved. Furthermore, the inter-diffusion and out-gassing between the materials involved in the fan-out WLP need to be controlled.